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Date: Thu, 16 Sep 2010 10:40:08 +0200
From: Andi Kleen <andi@firstfloor.org>
To: miaox@cn.fujitsu.com
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
	"Theodore Ts'o" <tytso@mit.edu>, Chris Mason <chris.mason@oracle.com>,
	Linux Kernel <linux-kernel@vger.kernel.org>,
	Linux Btrfs <linux-btrfs@vger.kernel.org>,
	Linux Ext4 <linux-ext4@vger.kernel.org>
Subject: Re: [PATCH] x86_64/lib: improve the performance of memmove
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diff --git a/arch/x86/lib/memcpy_64.S b/arch/x86/lib/memcpy_64.S
index bcbcd1e..6e8258d 100644
--- a/arch/x86/lib/memcpy_64.S
+++ b/arch/x86/lib/memcpy_64.S
@@ -141,3 +141,28 @@ ENDPROC(__memcpy)
 	.byte .Lmemcpy_e - .Lmemcpy_c
 	.byte .Lmemcpy_e - .Lmemcpy_c
 	.previous
+
+/* 
+ * Copy memory backwards (for memmove)
+ * rdi target
+ * rsi source
+ * rdx count
+ */
+
+ENTRY(memcpy_backwards):
+	CFI_STARTPROC
+	std
+	movq %rdi, %rax
+	movl %edx, %ecx
+	add  %rdx, %rdi
+	add  %rdx, %rsi
+	shrl $3, %ecx
+	andl $7, %edx
+	rep movsq
+	movl %edx, %ecx
+	rep movsb
+	cld
+	ret
+	CFI_ENDPROC
+ENDPROC(memcpy_backwards)
+	
diff --git a/arch/x86/lib/memmove_64.c b/arch/x86/lib/memmove_64.c
index 0a33909..6c00304 100644
--- a/arch/x86/lib/memmove_64.c
+++ b/arch/x86/lib/memmove_64.c
@@ -5,16 +5,16 @@
 #include <linux/string.h>
 #include <linux/module.h>
 
+extern void asmlinkage memcpy_backwards(void *dst, const void *src,
+				        size_t count);
+
 #undef memmove
 void *memmove(void *dest, const void *src, size_t count)
 {
 	if (dest < src) {
 		return memcpy(dest, src, count);
 	} else {
-		char *p = dest + count;
-		const char *s = src + count;
-		while (count--)
-			*--p = *--s;
+		return memcpy_backwards(dest, src, count);
 	}
 	return dest;
 }